A variety of insulating heat-resistant materials suitable for use in casting of non-ferrous metals are well known in the prior art. Of the insulating heat-resistant materials utilized in the process of casting non-ferrous metals that are known in the prior art, calcium silicate based materials have proven to be of particular utility because of their small heat capacities, high heat insulating capability and non-wetting properties in contact with molten non-ferrous metals.
Calcium silicate based insulating materials employed in casting of non-ferrous metals have typically been of the tobermorite-type matrix structure and xonotlite-type matrix structure of calcium silicate insulating material.
A fundamental tobermorite-type matrix structure of calcium silicate insulating material is disclosed in U.S. Pat. Nos. 4,111,712 and 4,128,434 to Pusch. This fundamental tobermorite type matrix structure of calcium silicate insulating material is produced by combining, a source of calcium, such as hydrated lime or quick lime, a source of siliceous material, such as silica, diatomaceous earth, silica fume, colloidal silica, or other suitable oxides of silicon, fibrous wollastonite and an organic fiber, such as kraft made from wood pulp, in the presence of at least one part by weight water per part by weight of the forgoing combined materials, to form an aqueous slurry. The aqueous slurry is then poured into a mold where the excess water is pressed out of the mixture to form an uncured shape, typically a sheet. The uncured shape is then placed in an autoclave where it is heated under steam pressure of about 100 psi. The shape is then oven dried to about 250 degrees Fahrenheit, and subsequently heat treated to above 500 degrees Fahrenheit. Finally, the resultant tobermorite type calcium silicate insulating material is cut or machined to the appropriate dimensions for use in the particular application.
As with the tobermorite-type matrix structure of calcium silicate insulating material, the xonotlite-type matrix structure of calcium silicate insulating material is known in the prior art. A fundamental xonotlite-type matrix structure of calcium silicate insulating material is produced by combining a source of calcium, such as hydrated lime or quick lime, a source of siliceous material, such as silica, diatomaceous earth, silica fume, colloidal silica, or other suitable oxides of silicon, fibrous wollastonite, an organic fiber, such as kraft made from wood pulp in the presence of at least one part by weight water per part by weight of the forgoing combined materials in an autoclave under about 200 psi steam pressure. The resultant aqueous slurry is then pressed in a mold and dried in an oven.
Alternatively, the fundamental xonotlite-type matrix structure of calcium silicate insulating material may be produced by mixing a source of calcium, such as hydrated lime or quick lime, a source of siliceous material, such as silica, diatomaceous earth, silica fume, colloidal silica, or other suitable oxides of silicon, fibrous wollastonite, an organic fiber, such as kraft made from wood pulp in the presence of at least one part by weight water per part by weight of the forgoing combined materials in the presence of water to form an aqueous slurry. The aqueous slurry is then poured into a mold where the excess water is pressed out of the slurry to form an uncured shape, typically a sheet. The uncured shape is then placed in an autoclave where it is heated under steam pressure of about 200 psi. The shape is then oven dried.
Finally, the resultant xonotlite type calcium silicate insulating material is cut or machined to the appropriate dimensions for use in the particular application.
Although these fundamental tobermorite type and xonotlite type calcium silicate insulating materials have been found to be suitable for use in connection with the casting of relatively low melting point non-ferrous metals and in other uses, certain shortcomings of these insulating materials have become apparent in application. In producing an optimal calcium silicate insulating material, it is desirable that the insulating material have reduced density, increased strength, improved thermal insulating properties, be homogeneous throughout with minimized thermal shrinkage. Of particular importance for calcium silicate insulating material utilized in connection with the casting of non-ferrous metals, such as aluminum, is the necessity that the material have sufficient physical strength. In casting non-ferrous metals, such as aluminum, the insulating material that comes in contact with the elevated temperature of the molten metal is particularly susceptible to cracking and fracture; therefore sufficient physical strength and thermal dimensional stability are required of the insulating material. Additionally, in connection with the casting of non-ferrous metals, it is desirable that outgassing of the insulating material in contact with the molten metal be minimized. Several variants and improvements of the tobermorite type and xonotlite type calcium silicate insulating materials are known in the prior art which attempt to rectify the shortcomings of the fundamental tobermorite type and xonotlite type calcium silicate insulating materials.
In the past, asbestos fibers had been utilized as a reinforcing fiber in manufacture of calcium silicate insulating materials to provide sufficient strength and toughness to the insulating material. Although such asbestos containing insulating materials performed well, the use of asbestos fibers has been widely discontinued due to health and environmental concerns.
U.S. Pat. No. 5,073,199 to Krowl et al. discloses a tobermorite type calcium silicate insulating material containing pitch based graphite fiber to provide toughness and strength to the insulating material. However, the incorporation of such graphite fiber and its associated material cost results in an appreciable increase in the cost of the resultant product.
U.S. Pat. No. 4,690,867 to Yamamoto et al. discloses a xonotlite type calcium silicate insulating material with improved strength suitable for non-ferrous metal casting wherein reinforcing carbon fibers are not uniformly distributed in the material thus having zones of varying strength. Use of the material disclosed in U.S. Pat. No. 4,690,867 for molten metal casting is often accompanied by undesirable outgassing which creates voids and contaminants in the resultant cast metal.
U.S. Pat. Nos. 4,773,470 and 4,897,294 to Libby, et al. disclose the use of delaminated vermiculite as a substitute for asbestos in the composition of a tobermorite insulating material suitable for use in molten metal casting. Although the use of vermiculite as a substitute for asbestos results in material with reduced thermal shrinkage in comparison to materials containing only wollastonite as the inorganic fiber, the machineability of the material is compromised.
As a final example of attempts of the prior art to rectify the shortcomings of the fundamental tobermorite type and xonotlite type calcium silicate insulating materials, U.S. Pat. No. 4,144,121 to Otouma, et al. and U.S. Pat. No. 4,334,931 to Assumi, et al. disclose the use of previously synthesized xonotlite crystalline material to provide strength comparable to that of an asbestos containing board. However, manufacture of these calcium silicate insulating materials is more costly, in that, an additional step is required to produce the xonotlite crystalline material that is incorporated with the starting materials.
Accordingly, it is the principle objective of the present invention to provide an insulating material that is suitable for use in non-ferrous molten metal casting that is lightweight with greater refractoriness, is tough and resistant to high temperature cracking, and does not possess the shortcomings of the prior art insulating materials.
An additional objective of the present invention is to provide an asbestos-free fire resistant, heat insulating, electrical insulating, and corrosion resistant material, that may be utilized in other applications in addition to non-ferrous metal casting, having reduced health exposure risk and minimal environmental impact.
Other objects and advantages of the present invention will be apparent to those skilled in the art from the following description of the invention.